1. Field of the Invention
The present invention relates to an optical reader, and in particular to an optical reader having an imaging device and a photographing optical system for reading images of an object.
2. Description of the Prior Art
Images that are obtained with optical microscopes or semiconductor inspection devices vary greatly depending on how the object is illuminated (differences in amount of light, wavelength, and angle of incidence). Differences in image are directly related to image scanning or inspection performance and reliability. Dark field illumination is an important illumination means for making minute objects easier to see or for improving processing efficiency in OCR or other applications.
Dark field illumination light sources are intended to illuminate objects at an angle of illumination that doesn't coincide with the optical axis of the photographing optical system. An illumination optical system including a ring-slit reflection component is used to illuminate an object from the peripheral direction thereof for dark field illumination (see Japanese Patent 3445722). Furthermore, optical readers are known in which light-emitting diodes (LED) are disposed with certain directional characteristics, i.e., are disposed in a pattern around the optical axis for use as ring illumination.
Semiconductor wafers are also an example of objects that are scanned by this type of optical reader. For example, this type of optical reader is sometimes used when symbols marked on a mirror-like surface are scanned for an OCR process or the like. Dark field illumination is also effective for applications such as the defect inspection of lenses or glass sheets. The illumination angle of the illuminating light may be important, but vary depending on the depth of the markings or defects, substances deposited above and around them or background thereof. Deposited substances may include oxide films, nitride films, or polyimides, and examples of background include circuit patterns.
As mentioned above, directional characteristics are required of dark field illumination light sources, and there is thus a need for dark field illumination devices that are capable of illumination at various angles to deal with defects/markings that vary in depth. In OCR optical systems which are used for wafer marking recognition, there is a need for an optical system that would be flexibly adaptable, since illumination angles of incidence suitable for a variety of wafer models would significantly enhance the recognition efficiency of individual wafer markings.
In optical readers for reading semiconductor wafer markings, characters and the like, an illumination light source of LEDs may be disposed at locations allowing the object to be illuminated at a desired angle of incidence in order to deal with defects/markings of various depths. In such cases, dark field illumination light sources must be disposed at multiple locations when illumination at multiple angles of incidence is required.
The general usability of a device will be expanded if the types of incident angles of dark field illumination are increased, but a resulting problem is that the number of illumination light sources is increased, and more space is needed for the light sources, thereby leading to devices of greater size and greater manufacturing costs. Furthermore, when the configuration of the dark field illumination is made variable, it is sometimes necessary to change the position of the illumination light source and change the direction of the illumination light source. Unless suitably constructed, the range within which the illumination light source can be moved will be limited, or the size of the device and the costs might possibly be needlessly increased.